1. Field of the Invention
The present invention relates generally to apparatus for generating a saw-tooth wave form, and more particularly to improved apparatus for generating a slope-adjusted saw-tooth wave for use in association with automatic extrusion apparatus.
2. Description of the Prior Art
In the manufacture of blow-molded bodies and articles, a parison is initially extruded, and thereafter inflated in a blow-mold to the desired shape. When designing the shape of the parison to be extruded, one must consider the strain to which the parison will be subsequently subjected during the blow-mold operation. To this end, parisons are formed to have varying wall thicknesses at different points along their extruded lengths. Known extruders have a pintle movable relative to a die head to create a variable orifice-like annular opening therebetween. Hence, when the pintle is moved further away from the die head, the parison wall thickness will be increased, and vice versa. Apparatus has also been developed to produce desired pintle movement in response to an electrical command signal. For example, an electrohydraulic servoactuator may be used to convert an electrical input command signal in to a corresponding hydraulic signal, which may in turn be used to cause such desired pintle movement.
It is also known to provide an electrical signal in the form of a plurality of superimposed voltage triangles, such as shown in our prior pending application Ser. No. 000,507, filed Jan. 2, 1979, or in U.S. Pat. No. 3,865,528. In effect, the ascending side of one triangle is superimposed on the descending side of another triangle so that the sum of their values will be a constant at any point in time. A potentiometer can be used to tap off an intermediate signal. Hence, a plurality of potentiometers each representing different positions of extruded parison length, can be individually set so as to produce a control signal reflective of the desired parison shape.
In known extrusion apparatus, such as that disclosed in our aforesaid pending U.S. patent application Ser. No. 000,507, the operation of a program is slaved to the position of an extrusion ram. Hence, a matrix representing parison wall thickness as a function of length, is scanned at a rate coupled to the rate at which the ram extrudes material. However, in alternate structure, such as that in which the plastic material is extruded by a continuously rotating screw conveyor, a ramp signal must be independently generated, and then desirably slaved to a system parameter. The object is to scan the parison wall thickness program at a rate proportional to the rate at which material is extruded. Such slaving will insure time synchronization between program sweep and parameter operation.